Ying is a professional deep water free diver. His altitude (in meters relative to sea level), $x$ seconds after diving, is modeled by: $D(x)=\dfrac{1}{36}(x-60)^2-100$ How many seconds after diving will Ying reach his lowest altitude?
Ying's altitude is modeled by a quadratic function, whose graph is a parabola. The lowest altitude is reached at the vertex. So in order to find when that happens, we need to find the vertex's $x$ -coordinate. The function $D(x)$ is given in vertex form. The vertex of $\dfrac{1}{36}(x-{60})^2{-100}$ is at $({60},{-100})$. In conclusion, Ying will reach his lowest altitude $60$ seconds after diving.